Re: drivers/resctrl/mpam_devices.c:1860:35-38: WARNING: Suspicious code. resource_size is maybe missing with msc_res

[Ilpo Järvinen]

On Thu, 3 Sep 2026, kernel test robot wrote:

> Hi Ben,
> 
> First bad commit (maybe != root cause):
> 
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   89a312991dc6e638a36adc43ccb91dbc25504c04
> commit: c544f00a473239835d22e7109b403314d8b85974 arm64: mpam: Drop the CONFIG_EXPERT restriction
> date:   5 months ago
> config: arm64-randconfig-r051-20260902 (https://download.01.org/0day-ci/archive/20260903/202609030809.ObirDhR3-lkp@xxxxxxxxx/config)
> compiler: clang version 24.0.0git (https://github.com/llvm/llvm-project 2c2cffa015e9549aafba9e514384b08204c7cc4b)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Fixes: c544f00a4732 ("arm64: mpam: Drop the CONFIG_EXPERT restriction")
> | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202609030809.ObirDhR3-lkp@xxxxxxxxx/
> 
> cocci warnings: (new ones prefixed by >>)
> >> drivers/resctrl/mpam_devices.c:1860:35-38: WARNING: Suspicious code. resource_size is maybe missing with msc_res
> 
> vim +1860 drivers/resctrl/mpam_devices.c
> 
> f04046f2577a5c James Morse 2025-11-19 @1860  		msc->mapped_hwpage_sz = msc_res->end - msc_res->start;

> :::::: The code at line 1860 was first introduced by commit
> :::::: f04046f2577a5c76167333ca99d3903ee5331ba0 arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate
> 
> :::::: TO: James Morse <james.morse@xxxxxxx>
> :::::: CC: Catalin Marinas <catalin.marinas@xxxxxxx>

Hi all,

This report has some validity but the code doesn't look buggy as is, just 
off-by-one errors correcting each other. The variable is called "..._sz" 
(~ size), but the resource size calculation is off-by-one because trying 
to do it by hand instead of using the resource_size() helper.

The mapped_hwpage_sz value is only used for register write offset 
sanity checks, which use > countering the off-by-one error.

IMO, it would be better to use resource_size() and change those sanity 
checks to use >= instead of > but this doesn't look a major problem by 
any means.

-- 
 i.